The present invention relates to a release sheet having a release layer formed on at least one side of a single-layered or laminate support sheet with an undercoated layer inserted between the release layer and support
Various types of release sheet are used with adhesive products such as adhesive tape and labels, as well as for producing synthetic leathers and shaped articles such as synthetic polyvinyl chloride leathers and carbon fiber prepregs. While release sheet used as an adhesive product should protect the adhesive surface of the product, it must be easily releasable from the sheet prior to use. Release sheet used in the production of synthetic leathers and shaped articles must also have good releasability, but in addition, the sheet must be heat-resistant and have a sheet strength sufficient to withstand handling in a specific process step. The basic construction of the release sheet used for these purposes consists of a support sheet having a release layer formed on at least one side thereof, with an undercoated layer interposed between the support and the release layer.
The support sheet is usually made of a cellulosic substrate such as paper, an inert polymeric film, a metal foil or a lamination thereof. Polymeric films having an inert surface have limited uses if they are not combined with other substrate materials since it is difficult to obtain a desired degree of releasability with the polymeric film alone. Furthermore, there are no undercoating agents that provide strong adhesion between polymeric films or metal foils and release agents. In order to provide better adhesion, attempts have been made to treat the surface of metal foils or polymeric films by corona treatment but no satisfactory results have been attained except with polyethylene films.
Release sheet is frequently subjected to various kinds of heat during processing or use, and the undercoating agent must be sufficiently heat-resistant to cause no change in the adhesion between the support sheet and the release agent.
Cellulose bases are able to possess a desired degree of releasability if a proper type of release agent is used, and a consistent releasability can be ensured by forming a uniform undercoated layer on the base surface. Among various cellulose bases, paper is extensively used. Most of the undercoated layers used today are formed by (1) a clay-base pigment, (2) a water-soluble polymer such as poly(vinyl alcohol) or (3) a polyethylene extrudate. However (1) has poor adhesion to the release agent, (2) shrinks greatly with heat, and (3) is not highly heat-resistant.
As shown above, conventional release sheet products have various defects, and none of the one-faced or double-faced release sheets available today have the properties required of various adhesive products or the enhanced processing necessary in the manufacture of synthetic leathers or shaped articles.